It's all beginning to fall into place - or getting more confusing, not sure which.
I had a sneaking suspicion that Joseph may have been married before and the younger kids on the 1851 weren't Sophia's, especially as they didn't marry until 1850. Maybe Elizabeth Bays/Bass was Sophia's from a previous relationship.
Anyway, found these baptisms on Famlysearch, they match the younger kids on the 1851
All with parents Joseph Darnell & MARY
William Darnell bapt 14 Aug 1836 Sandy
Eliza Darnell bapt 28 Sept 1841 Sandy
Ann Darnell bapt 10 Sept 1838 (she wasn't on the 1851)
Then there's this
Mary Ann Darnell bapt 10 July 1853 Wesleyan, Biggleswade Circuit, Bedford.
Parents Joseph Darnell and SOPHIA
Either there is a parallel family out there with a similiar surname or our Darlows didn't know what to call themselves.